The recently described Cryphonectriaceae (Diaporthales) includes numerous important tree pathogens such as the chestnut blight pathogen Cryphonectria parasitica, and various species of Chrysoporthe that cause serious stem canker diseases on Eucalyptus. A recent investigation of dying Eucalyptus grandis clonal hedges in northern Sumatra, Indonesia, revealed the presence of an unknown member of the Cryphonectriaceae. DNA sequence comparisons with other members of the Cryphonectriaceae showed that the Indonesian fungus represents a new genus and species in the family, for which we provide the name Cryptometrion austuescens gen. sp. nov. It can be distinguished from other Cryphonectriaceae on Eucalyptus, such as Chrysoporthe, Microthia, Holocryphia and Cryphonectria, based on its orange, limited stromatic tissue, single septate, fusoid to ellipsoid ascospores and the absence of paraphyses among the conidiogenous cells in the anamorph. Inoculations with this fungus on two clones of E. grandis showed that it is highly pathogenic and has the capacity to cause serious losses to Eucalyptus plantations in the region.